Magnetic (or magnetoresistive) random access memory (MRAM) using magnetoresistive (MR) elements (or magnetic tunnel junctions (MTJs)) is a strong candidate for providing a dense, scalable and non-volatile storage solution for future memory applications. MRAM is capable to provide a high speed writing and low operation power. In particular, these parameters can be achieved in the MRAM employing a spin-induced writing mechanism in MTJ with a perpendicular anisotropy.
Each MTJ (or MR element) of MRAM comprises at least a pinned (reference) magnetic layer with fixed direction of magnetization and a free (storage) magnetic layer with reversible direction of the magnetization. The free and pinned ferromagnetic layers are separated from each other by a thin tunnel barrier layer made of insulator or semiconductor. The free layer works as a storage layer and can have two stable directions of the magnetization that is parallel or anti-parallel to the direction of the magnetization of the pinned layer. Resistance of the MTJ measured in a direction across the tunnel barrier layer thickness depends on a mutual orientation of the magnetization directions in the free and pinned layers. It can be effectively controlled by a direction of the spin-polarized switching current running across the MTJ perpendicular to tunnel barrier layer plane. The spin-polarized current can reverse the magnetization direction of the free layer. The resistance is low when the magnetization directions of the free and pinned layers are parallel to each other (logic “0”) and high when the magnetization directions are antiparallel to each other (logic “1”). Difference in the resistance between two logic states can be in a range of about 1000% at room temperature.
A typical MRAM device includes an array of memory cells, word lines extending along rows of the memory cells and bit lines extending along columns of the memory cells. Each memory cell is located at a cross point of a word line and a bit line in a vertical space between them. The cell typically comprises an MTJ and a selection transistor (or other nonlinear element) connected in series.
The MTJ comprising magnetic layers made of materials having a perpendicular anisotropy (or perpendicular direction of magnetization in an equilibrium state) can provide a substantial thermal stability (Δ≧60). The thermal stability can be estimated from the following equationΔ=KUV/kBT,   (1)where KU and V is a magnetic anisotropy and volume of the free layer, respectively, kB is a Boltzmann constant, and T is a temperature.
Low density of the spin-polarized switching current (JS≦1·106 A/cm2) and high tunneling magnetoresistance (TMR≧150%) originate from a coherent spin-dependent tunneling of highly spin-polarized electrons. They can be achieved in the perpendicular MTJ comprising a coherent body-centered (bcc) structures with (001) planes oriented in the adjacent free, tunnel barrier and pinned layers. The coherent bcc (001) structure can also provide a low magnetic damping constant (α≦0.01).
Free layer of the perpendicular MTJ can be made of magnetic materials having a substantial perpendicular magnetic anisotropy (PMA). There are several groups of the perpendicular magnetic materials which include ordered alloys (FePt, CoPt, FePd, and the like), rare earth-transition metal (RE-TM) alloys (GdFeCo, TbFeCo, TbCo, and the like), and laminates (CoFe/Pd, CoFeB/Pt, Co/Pt, Co/Pd, and the like). These materials can suffer from a significant damping constant α≧0.01 and reduced spin polarization p which cause a substantial increase of the switching current density JS. Some of the these materials cannot provide the required coherent bcc (001) structure of the MTJ, or cannot tolerate a high temperature annealing (TAN≧250° C.) due to possible loss of the perpendicular anisotropy or unwanted diffusion in the tunnel barrier layer.
CoFeB/MgO/CoFeB multilayer became a system of choice for MTJ manufacturing since it can provide the required coherent bcc (001) texture in MgO and adjacent crystallized CoFe(B) layers. However this structure may not support the perpendicular direction of the magnetization in the free and pinned layers. The PMA in the CoFeB-based free and pinned layers can be provided by a magnetic surface anisotropy produced at the interfaces of the CoFeB layers with other materials, for example MgO. However a magnitude of the PMA produced at the CoFeB/MgO interface may be not sufficient to support the perpendicular direction of the magnetization in the crystallized CoFe(B) layer having a thickness of about 1 nm and above. Recent demonstrations of the PMA in the annealed Ta/CoFeB/MgO multilayers provided a possibility to increase thickness of the free layer up to about 1.3 nm. This thickness may be not sufficient for providing the required thermal stability of the perpendicular MTJs having a diameter less than 40 nm.